At our old house Zoe would have a spaz attack when we'd go to bed each night. Even though EVERY light was out she's stare through the open bedroom window down the stair into nothingness and bark and bark and bark, like there was something there she was upset/afraid of. Our solution: close the bedroom door as soon as we could and she wouldn't bark at the closed door.
In our new house the street light can be seen a bit through the verticals of our bedroom and yet she never barks at anything! I just don't know what the fuss was about at the old house but not at the new house
I think since our babies are sensitive (to noises, etc) they can detect things we can't. It may also be a stage thing, like mimimomo suggested.